Centrifugal pumps are perhaps the most common type of pump in operation today. With many different configurations available, centrifugal pumps are widely-used because of their design simplicity, high efficiency, wide range of capacity and head, smooth flow rate and ease of operation and maintenance. Centrifugal pumps use one or more impellers, which attach to and rotate with the pump shaft. This provides the energy that moves fluid through the pump and pressurizes the fluid to move it through a piping system. The pump therefore converts mechanical energy from a motor to energy of a moving fluid. A portion of the energy goes into kinetic energy of the fluid motion, and some goes into potential energy, represented by fluid pressure or by lifting the fluid, against gravity, to a higher altitude. As used herein, the term fluid is intended to encompass liquids and gases of varying densities, as well as liquids and gases containing solids. The matter flowing through a pump is also called the pumpage.
A centrifugal pump works by directing fluid in the system into the suction port of the pump and from there into the inlet of the impeller. The rotating impeller then moves the fluid along the spinning vanes, at the same time increasing the velocity energy of the fluid. The fluid then exits the impeller vanes and moves into the pump volute or diffuser casing, where the velocity of the fluid is converted into pressure through a diffusion process. The fluid is then guided into the discharge port of the pump and from there out into the system, or on to the next stage in the case of a multi-stage pump.
Centrifugal pumps are used in a variety of circumstances and conditions. They are often used for lower viscosity fluids and high flow rates. However, they may also be used with moderate and higher viscosity fluids or pumpage containing solids. They are typically used across many residential, commercial, industrial, and municipal applications. For example: commercial and residential building services, including pressure boosting, heating systems, fire protection sprinkler systems, drainage, and air conditioning; industry and water engineering, including boiler feed applications, water supply (municipal, industrial), wastewater management, irrigation, sprinkling, drainage and flood protection; chemical and process industries, including chemicals, hydrocarbons, pharmaceuticals, cellulose, petro-chemicals, sugar refining, food and beverage production; and secondary systems, including coolant recirculation, condensate transport, cryogenics, and refrigerants—to name a few.
It should be appreciated that no single centrifugal pump will meet all needs. It should also be appreciated that under current state of the art practices, it is impractical for pump manufacturers to design and build a custom pump for each customer's particular end use application. Rather, most pump manufacturers will offer a finite line of pumps offering varying performance characteristics. The line of pumps typically comprises a number of differently sized pump casings, a number of differently sized pump motors, and typically a single impeller design that can be used with each pump casing. The impeller may be modified, such as by trimming its vanes, to shift the performance characteristics of an individual pump. In addition, there is a practical limit to the number of different pump casings, pump motors and impellers a manufacturer will stock. Accordingly, situations arise where a specific pump manufacturer does not stock a pump that meets the needs of a specific customer's end use application. For example, available impeller, motor and pump casings may not achieve the desired flow rate and/or head requirements, or these requirements may be achieved but at a low or unacceptable efficiency. As a result, the manufacturer will attempt to trim an existing impeller to meet the customer's performance needs. If the manufacturer is unable to modify the impeller in a way that achieves the customer's requirements, the customer may ultimately purchase the pump from another manufacturer and the first contacted manufacturer loses a sale.
Many factors are important when designing or selecting a pump. Among these factors are efficiency, flow rate and head. The importance of pump efficiency is directly related to the use of energy and, therefore, cost to operate. Friction produced by bearings and other mechanical components, such as seals, stuffing box, etc., adversely affect pump efficiency, but the impeller and volute have the greatest influence on efficiency. For any given impeller, it is known that the head it produces varies as the square of a change in speed. Generally speaking, head is the height at which a pump can raise a fluid. Double the speed and the head increases by a factor of four. If the speed is held constant, the same rule holds true for a change in its diameter. In other words, double the diameter of the impeller and the head increases by a factor of four. The fluid flow through an impeller follows a similar rule but its change is directly proportional to the impeller speed or diameter. Accordingly, doubling the speed or diameter of the impeller doubles the fluid flow. A change in rotational speed of an impeller is in reference to the peripheral speed of a point on its outer most circumference. It is this speed that determines the absolute maximum head and flow attainable by any impeller.
The head produced by an impeller is almost entirely dependent upon its peripheral velocity but, flow is influenced by several other factors. The width and depth of the vanes and the diameter of the impeller center opening or eye are important considerations as they determine the ease with which some volume of water can pass through the impeller. Other factors such as vane shape also influence an impeller's performance.
The shape and spacing of the impeller vanes also have a large effect upon efficiency. Ideally, a pump would have as many vanes as possible that fit within the casing, but the physical constraints of the casing typically limits the number of vanes to between 5 and 7 and even fewer for pumps that handle larger solids.
Deeper vanes will produce high flow. Conversely, shallower vanes and deep expeller vanes will increase head. Any pumping application is balancing between obtaining the flow required at the correct head. Adding depth to the pumping vane will increase overall flow, but possibly drop the pressure capability. Adding depth to the expeller vane will increase the overall pressure capability but possibly drop flow capacity. Deeper expeller vanes can also increase head by a reduction of pressure applied to the seal. Putting aside designing and manufacturing a custom impeller for each end user's specific needs which would be costly and take too long to produce, current pump manufacturers have, at best, a limited ability to vary the relative depth of pumping vanes and expeller vanes, and almost no ability to vary vane curvature, vane configuration or orientation, or vane count. To the contrary, present manufacturers typically are limited to a finite and small number of fixed impeller designs for each pump casing sell. As used herein, the terms vane or impeller vane refers to the vanes on the same side of the back plate as the intake port, and the term expeller vane refers to the vanes on the side of the back plate opposite the intake port.